This invention relates to exhaust flow nozzles, such as those used in gas turbine engines, having serrations or undulations to reduce noise.
The generation of noise from turbulent jet exhausts is of significant practical interest for low and moderate bypass ratio engines used in subsonic civil transports. The jet exhaust noise is one component of overall engine noise, and is particularly important at take-off and cutback conditions. For high bypass ratio engines, the jet noise contribution is reduced, but it is still a factor especially with continually tightening of noise restrictions.
Prior approaches to jet noise reduction have relied primarily on mixing enhancement, where the aim is to promote the exchange of momentum between the high-speed primary stream and the lower-momentum secondary flow (i.e. fan bypass and/or ambient flight stream). Tabs and chevron-type devices have been used for single stream and separate flow exhaust systems. Lobed mixers have been used for mixed-flow exhausts.
An inherent shortcoming of the aforementioned mixing devices is their tendency to generate parasitic high-frequency noise. Thus, while low-frequency noise reduction can readily approach 2-4 dB in noise spectra, reductions in overall (community) noise metrics such as Effective Perceived Noise Level (EPNL) are appreciably lower. In some instances, the high frequency noise penalty can completely offset low-frequency reduction, resulting in increased EPNL.
A second shortcoming of external plume mixing devices is adverse impact on aerodynamic performance (axial thrust). Tabs and chevrons typically increase Total Specific Fuel Consumption (TSFC), and can adversely affect nozzle discharge characteristics at off-design points in the flight envelope.